The electrorheological (ER) fluid is made of nano-particles or micro-particles suspended in insulating liquid. The shear strength of the fluid may be continuously adjusted electrically, and the material may undergo liquid to solid transition within milliseconds. The outstanding characters of the fluid, including its continuously adjustable shear strength, quick response, and reversible transition, make it an intelligent material with tunable hardness having broad and important applications. The material may be used in the clutch, damping system, damper, braking system, automatic transmission, liquid valve, mechanoelectrical coupling control, robotics, etc., making it possible consolidated intelligent mechanoelectrical control. The material may be applied in almost all industrial and technological fields and has wide application in military fields.
Since the ER fluid was invented by Winslow in the 1940s, however, the material has not been widely applied as expected. The lack of application is due to its relatively low shear strength, usually about several kPa and 10 kPa at the most, high leakage current, and tendency towards settling. The working principle of the ER fluid is generally as follows: in an electric field, particles are polarized and become attracted to each other, the shear strength increases as the intensity of the electric field increases. The ER fluid based on the attraction of the polarized particles is referred to as the “ordinary ER fluid” or “dielectric ER fluid.” The upper limit in the shear yield strength for this type of material is 10 kPa (1 kV/mm). Such low shear strength makes it impossible meeting the requirements for technological and industrial applications. In the late 1990s, Institute of Physics, Chinese Academy of Sciences, invented surface modified complex strontium titanate ER fluid (Chinese Patent No. CN 1190119), which may has the shear yield strength up to only 30 kPa at an electric field of 3 KV/mm.
Current literature and patents mostly disclose the material and technology of the ordinary ER fluid. CN1490388 discloses an ER fluid made of urea-coated barium titanate nanoparticles called the giant ER fluid. The patent discloses complex particles and a promoter which contains urea, butyramide, and acetamide. The static yield strength of the giant ER fluid may reach 130 kPa due to the coating layer surrounding the surface of the particles. The theoretical basis is named the theory of Coating Layer Saturated Polarization. The main drawbacks of the giant ER fluid are the necessity of the surface coating of the particles, high current density (several hundred μA/cm2) as reported, low yield strength at low electric field, e.g., only 30-40 kPa at 2 kV/mm, and the phase transition of barium titanate at around 120° C. All of the drawbacks restrain the application of the material. A doped titanium oxide ER fluid and method for preparing the same have been reported in the literature. The doped micro- or nano-particles of titanium dioxide are prepared by mixing highly polarized molecules of amides or their derivatives in titanium dioxide via the sol-gel method. Then, the doped particles are mixed with methyl silicon oil at a volume percentage of 30% to obtain a high yield strength ER fluid. CN1752195 discloses a calcium titanate ER fluid and method for preparing the same. The composition mainly consists of an anhydrous calcium titanate ER fluid. The ER fluid is prepared by preparing calcium titanate particles via oxalic acid co-precipitation and mixing the prepared particles with dimethyl silicon oil at a volume percentage of 30%. The ER fluid exhibits strong ER effect, its yield strength may reach more than 100 kPa. However, these ER fluids can not be widely applied due to their high current leakage density and limitations on the preparation material.